If you wanted to replace that one speaker with another one, just go to the wreckers and pick one up. Its not too hard, just gotta take off the door trim and unscrew the speaker, then unplug the old and plug in the new
If you wanted to upgrade to aftermarket speakers, you'll need a pair of 6" (6.5" is a little too big and you would have to modify the speaker mounting a little). Obviously you'll need to take both door trims off, as well as cutting the old speaker plugs off or getting some spade terminals to connect to your new speakers.
Heres some examples of what is a "drop in" replacement for your speakers.